For many years, a pair of superimposed rotary dies with cutting blades on one of both cylinders have been used to cut blanks from a thin web of material passing through the nip of the dies. Typically, the thin web is of a material such as paper, paper board, cardboard, plastic film, metal foil, thin sheet metal and the like.
U.S. Pat. No. 4,608,895 discloses a pair of rotary die cylinders with integral complementary cutting blades thereon which coact to sever a web of material passing through their nip. Each severing blade has an elongate land projecting generally radially outwardly from its cylinder and having in cross section an outer face and spaced apart depending side faces inclined towards each other at an acute included angle, and each inclined to its associated outer face at an obtuse included angle. When the line of severance subtends more than a quandrant, the cutting edge transfers from one side edge to the other of the coacting lands at a pair of cross over sections each having an outer face which is substantially narrower than the outer face of the adjacent portion of the land. To enable proper registration of the coacting lands to cut a web, the pair of die cylinders can be relatively shifted both axially and in rotational phase with respect to each other.
The die is made from a cylindrical blank of tool steel which is hardened and ground to produce a cylindrical surface before it is electrical discharge machined (EDM) to produce the lands with the desired geometry thereon. To produce the desired accuracy and geometry of the lands, they are EDM machined in the hardened cylindrical workpiece utilizing a negative electrode, which is preferably cylindrical with grooves machined in the periphery thereof with a generally V-shape cross section for forming the lands of the die cylinders.
While these die cylinders have been sold commercially and performed satisfactorily for relatively high volume mass production operations, they are expensive and difficult both to manufacture and resharpen when they become worn or dull in use.